1. Field of the Invention
The present invention relates to a high pressure fuel supply apparatus chiefly for use in a cylinder fuel injection engine or the like.
2. Description of the Related Art
FIG. 6 is a configuration diagram showing a fuel supply system in an internal combustion engine for a vehicle, including a related-art high pressure fuel supply apparatus. In FIG. 6, fuel 2 in a fuel tank 1 is delivered from the fuel tank 1 by a low pressure pump 3, passed through a filter 4, adjusted in pressure by a low pressure regulator 5, and then supplied to a high pressure fuel supply apparatus 6 which is a high pressure pump. Only a flow rate of the fuel 2 required for fuel injection is boosted by the high pressure fuel supply apparatus 6, and supplied into a delivery pipe 9 of an internal combustion engine not-shown. A surplus of the fuel 2 is relieved between a low pressure damper 12 and a suction valve 13 by an electromagnetic valve 17.
In addition, the required fuel flow rate is determined by a control unit not-shown, which also controls the electromagnetic valve 17. The high pressure fuel supplied thus is injected into a cylinder of the internal combustion engine in the form of high pressure mist from a fuel injection valve 10 connected to the delivery pipe 9. When abnormal pressure (high relief valve opening pressure) is placed in the delivery pipe 9, a filter 7 and a high pressure relief valve 8 are opened to prevent the delivery pipe 9 from being broken.
The high pressure fuel supply apparatus 6 which is a high pressure pump, has a filter 11 for filtering the supplied fuel, a low pressure damper 12 for absorbing the pulsation of the low pressure fuel, and a high pressure fuel pump 16 for pressurizing the fuel supplied through the suction valve 13 and discharging the high pressure fuel through a discharge valve 14.
FIG. 7 is a longitudinal sectional view showing a related-art high pressure fuel supply apparatus. In FIG. 7, the high pressure fuel supply apparatus 6 has a casing 61, a high pressure fuel pump 16, an electromagnetic valve 17, and a low pressure damper 12, integrally, wherein the high pressure fuel pump 16 is a plunger pump provided in the casing 61.
A fuel pressurizing chamber 163 surrounded by a sleeve 160 and a plunger 161 inserted slidably in the sleeve 160 is formed in the high pressure fuel pump 16. The other end of the plunger 161 abuts against a tappet 164 shaped like a closed-end cylinder, and the tappet 164 abuts against a cam 100 as a driving unit to drive the high pressure fuel pump 16. The cam 100 is provided integrally or coaxially with a cam shaft 101 of the engine so as to reciprocate the plunger 161 along the profile of the cam 100 in cooperation with the rotation of a crank shaft of the engine. The volume of the fuel pressurizing chamber 163 is changed by the reciprocating motion of the plunger 161 so that the fuel boosted to high pressure is discharged from the discharge valve 14.
In the high pressure fuel pump 16, a plate 162, the suction valve 13 and the sleeve 160 are held between the casing 61 and an end surface of a spring guide 165, and fastened with a bolt 180. The plate 162 forms a fuel suction port 162a for sucking fuel from the low pressure damper 12 to the fuel pressurizing chamber 163, and a fuel discharge port 162b for discharging the fuel from the fuel pressurizing chamber 163.
The suction valve 13 shaped into a thin plate is formed in the fuel suction port 162a. The discharge valve 14 is provided on the fuel discharge port 162b so as to communicate with the delivery pipe 9 through a high pressure fuel discharge passageway 62 provided in the casing 61. In addition, in order to suck fuel, a spring 167 for pushing the plunger 161 down in a direction to expand the fuel pressurizing chamber 163 is disposed in the state where the spring 167 has been compressed between the spring guide 165 and a spring holder 168.
The electromagnetic valve 17 has an electromagnetic valve body 170, a valve seat 173, a valve 174, and a compression spring 175. The electromagnetic valve body 170 is incorporated in the casing 61 of the high pressure fuel supply apparatus 6 so as to have a fuel channel 172 inside the electromagnetic valve body 170. The valve seat 173 is provided in the fuel channel 172 of the electromagnetic valve body 170. The valve 174 is held on/off the valve seat 173 in the electromagnetic valve body 170 so as to close/open the fuel channel 172. The compression spring 175 presses the valve 174 onto the valve seat 173.
At a point of time when a flow rate requested from a control unit not-shown has been discharged in a discharge stroke of the high pressure fuel pump 16, a solenoid coil 171 of the electromagnetic valve 17 is excited to open the valve 174. Thus, the fuel 2 in the fuel pressurizing chamber 163 is released to the low pressure side between the low pressure damper 12 and the suction valve 13 so that the pressure in the fuel pressurizing chamber 163 is reduced to be not higher than the pressure in the delivery pipe 9. Thus, the discharge valve 14 is closed. After that, the valve 174 of the electromagnetic valve 17 is opened till the high pressure fuel pump 16 proceeds to a suction stroke. The timing to open the electromagnetic valve 17 is controlled so that the amount of fuel discharged into the delivery pipe 9 can be adjusted.
However, the related-art high pressure fuel supply apparatus has problems as follows. FIGS. 8A to 8C are enlarged views of the vicinity of the abutment portion between the plunger 161 and the tappet 164 in the high pressure fuel pump of the related-art high pressure fuel supply apparatus. FIG. 8A is a longitudinal sectional view, FIG. 8B is a sectional view taken on line A—A, and FIG. 8C is a bottom view. In addition, FIG. 9 is a graph showing the surface pressure distribution in the abutment surface between the tappet and the cam. In FIG. 9, the ordinate designates the surface pressure (MPa), and the abscissa designates the axial length of the cam shaft. The solid line shows the surface pressure distribution at the time of high fuel pressure (15 MPa), and the broken line shows the surface pressure distribution at the time of low fuel pressure (7 MPa).
As shown in FIGS. 8A to 8C, the tappet 164 has a circular abutment surface against the plunger 161 so that the tappet 164 abuts against the plunger 161 all over the abutment surface. In this case, the surface pressure distribution appearing in the abutment surface between the tappet 164 and the cam 100 shows a mountain-like shape taking a peak value in its central portion as shown in FIG. 9. In the comparatively low fuel pressure (e.g. 7 MPa), the deformation of the tappet 164 is limited to small deformation due to the rigidity of the bottom portion of the tappet 164 depending on the board thickness thereof so that the surface pressure distribution becomes comparatively flat. On the other hand, in the high fuel pressure (e.g. 15 MPa), the deformation of the bottom portion of the tappet 164 is so great that the surface pressure distribution shows an obviously mountain-like shape, thereby causing a problem that the central portion of the tappet 164 maybe abraded. When the central portion of the tappet 164 is abraded, the cam 100 is also abraded. Thus, due to the shortness of the discharge quantity caused by the lowering of the cam lift, the engine may halt.
To solve such a problem, it can be considered that the thickness of the bottom portion of the tappet 164 is increased to reduce such deformation. However, there is a problem that the weight of the apparatus increases.
It can be also considered that the curvature radius of the cam 100 is increased to enlarge the contact area of the abutment portion of the cam 100 against the tappet 164 to thereby prevent the abrasion in the central portion of the tappet 164. However, there is a problem that the increased diameter of the cam 100 increases the scale and weight of the apparatus.
Further, measures taken to reduce the outer diameter of the plunger 161 to reduce the load with which the tappet 164 is pressed can be also considered. Adversely, the cam lift increases conspicuously so that the traveling speed of the plunger increases conspicuously. Thus, there is a problem that the plunger 161 is burnt.